1991 BMW M5 - Sleeper Cell

Under such deep cover, BMW won't even acknowledge its existence.

"Drive it like you stole it." Those may have been the five nicest words anyone's ever said to me. Whether they're the wisest words to say to someone who has just gotten behind the wheel of a twin-turbocharged, 3.9-liter, E34 M5 for the first time is debatable. Even so, I felt inclined.

Stepping onto the stiff clutch, comprised of a SPEC E46 M3 pressure plate and an M5 disc, and pushing and pulling through the heavy gearbox, the throws shortened with a Rogue shifter, the immediate impression is this is a man's car. As it's been stroked from 3.5 to 3.9 liters, there's plenty of torque down low to keep it drivable around town. The turbos are so seamlessly integrated to the powerband that it's impossible to tell when they start spooling. That is, until you really nail it.

But all hell doesn't necessarily break loose like with many turbocharged cars; instead, there's a relentless, linear surge of power all the way to its 7000-rpm redline. Huge swaths of terra firma are consumed through each gear while the rear tries to counter the weight transfer.

There seems to be no amount of wind resistance or weight liabilities that can stop it, even as the car begins to climb through the triple digits. And this is with the boost turned down to 9.5 psi during the break-in period.

The E34 is the forgotten M5. It didn't make quite the splash of its predecessor, the E28, or its successor, the E39. For a while, people didn't even know if BMW was going to bring it into the United States. And when they eventually did, only small numbers arrived. Spotting one on the road was about as likely as seeing Sasquatch riding a unicorn. Then came the 540i and its big V8, relegating the E34 M5 to near obsolescence.

The current owner, a die-hard BMW fan, has always admired the E34 for its understated design, and after he was tipped off that there was one for sale he jumped on it immediately. What makes this particular car unique is that the VIN number doesn't show up on BMW's database, leading to speculation that this may be one of a small batch of early, hand-built M5s never intended for sale to the public.

It's a Euro-spec model built in December 1991 that was imported and federalized by a German Air Force pilot stationed in New Mexico, but any history before that is still unknown. Shortly after buying the car, the owner blew a head gasket. He then took the car over to Jon Firpach at GoodSpeed Performance Lab in Scottsdale, Ariz., for repairs. He also wanted to put in a new chip and when they opened up the ECU, which was unlike the stock ECU, they found a few notes, written in German, wrapped in plastic and taped to the inside of the ECU box.

Firpach also noticed that the idle was particularly lopey, as if it had a hotter cam. This led to more speculation on the BMW forums that this car may also be one of the rumored 400-hp E34s that M Motorsport built strictly for in-house research. It is believed that those cars were painted the same diamond metallic color as this car here.

While looking over the engine, Firpach took some measurements for his own curiosity and realized that there was enough room for a turbo or two. He says he suggested a twin-turbo in jest, while the owner says Firpach tried to sell him on the idea of forced induction. Either way, a call to BMW engine guru Jim Row at Metric Mechanic was all it took for the owner to decide to go all-in. His only requirement was that engine not have any turbo lag.

Metric Mechanic already offered a 3.9-liter stroker program for the engine, which the owner originally intended to buy. Turbocharging it only required a redesign of the skirt on the forged pistons.

After the engine came back to GoodSpeed, Firpach bolted on a pair of GT2871 turbos to a custom stainless-steel exhaust manifold and 3-inch stainless downpipes. To get more air in, they modified the OEM intake manifold and also made two 3-inch ram-air intakes to feed the airbox. Firpach went with a liquid-to-air intercooler instead of an air-to-air unit that would've required cutting into the core support. Aside from a few stainless pipes, the engine bay looks next to OEM. The turbos are hidden discreetly down low, completely out of sight.

The engine uses a Proefi stand-alone ECU; an uprated, in-tank fuel pump and an Aeromotive fuel pressure regulator. Many of the hard-to-find OEM replacement parts were sourced through Saliya Ukuwela at Euro M Cars. The exhaust is a custom GoodSpeed fabrication utilizing E60 M5 cats and a Supersprint muffler and is completely emissions legal.

As mentioned earlier, I drove the car during the break-in period, so boost was set at 9.5 psi. Since then, they've turned it up to 16.5 psi and registered a corrected 682 crank horsepower on GoodSpeed's Maha dyno. This is with 91 octane pump gas. A high-boost race gas tune is in the works. Row at Metric Mechanic said he'd guarantee the engine to withstand up to 1,000 hp. Both the owner and GoodSpeed's Firpach say that with the boost at 16.5 psi, the power is now too much for words.

A Ground Control suspension setup keeps it planted to the ground, but because there was so much weight transfer to the rear, new, stiffer springs have replaced the ones I drove on. Powerflex bushings and Dinan sway bars replace the stock units. Eighteen-inch M Motorsport Parallel wheels from a 7 Series have replaced the Throwing Star five spokes. They're wrapped with Toyo Proxes RA1 tires, 245/40 in front, 275/35 in back.

One revealed weakness was that the Wilwood six-piston brake calipers simply weren't up to task. Although better than the stock setup, they started to fade after a few hard stops from high speeds during my brief drive. A pair of Brembo eight-piston calipers and carbon-composite discs from a Mercedes-McLaren SLR remedied the problem. A taller, Dinan 3.64 rear differential was also added, which Firpach says increases the load on the turbos and also lowers the revs for quieter highway cruising.

The owner has said he has yet to come up against a car that can run with him. To date, his list of victims includes a couple of E60 M5s, a Z06 Corvette and a GT3. Odds are, they thought he was easy prey and are probably still wondering what hit them. And it's that sleeper quality that appeals to him the most.